using System;

public class Program

{

    public static void Main()

    {

        A a = new A();

        if (a is B b) {

            Console.WriteLine("a is B");

        } else {

            Console.WriteLine("a is not B");

        }

        var isNotTekstAttribute = a is B bb;

        if (isNotTekstAttribute) {

            Console.WriteLine("a is B");

        } else {

            Console.WriteLine("a is not B");

        }

    }

}

​

public class A {

}

​

public class B : A {}